Rotating stereo sonar mapping and positioning system

ABSTRACT

A stereo sonar system comprising a pair of pulsed sonar transducers mounted one above the other on a stationary bottom frame. The transducers are rotated as a unit but are spaced sufficiently to produce a three dimensional figure when stereo viewed.

United States Patent [191 Malloy et al.

[ 51 Dec. 25, 1973 ROTATING STEREO SONAR MAPPING AND POSITIONING SYSTEMInventors: Richard J. Malloy, Ojai; Robert D.

Hitchcock, Ventura, both of Calif.

The United States of America as represented by the Secretary of theNavy, Washington, DC.

Filed: May 1, 1972 Appl. No.2 249,330

Assignee:

US. Cl 340/3 R, 340/5 MP, 340/8 S Int. Cl. G0ls 9/66 Field of Search340/3 R, 3 F, 5 R,

340/5 MP, 8 R, 8 S

[56] References Cited UNITED STATES PATENTS 3,161,255 12/1964 Balise,Jr. 340/8 S Primary Examiner-Richard A. Farley Attorney-Richard S.Sciascia, Q. B. Warner and J. M. St. Amand [57] ABSTRACT A stereo sonarsystem comprising a pair of pulsed sonar transducers mounted one abovethe other on a stationary bottom frame. The transducers are rotated as aunit but are spaced sufficiently to produce a three dimensional figurewhen stereo viewed.

4 Claims, 3 Drawing Figures TO SURFACE SHIP JAIENTEUBEE25 @915 3,781,775

TO SURFACE SHIP llllllll ROTATING STEREO SONAR MAPPING AND POSITIONINGSYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention.

This invention relates to devices for mapping the seafloor and forprecisely positioning specific objects on the seafloor. It is moreparticularly adapted to the use of stereo sonar imagery to achieve suchseafloor mapping.

2. Description of the Prior Art.

Heretofore, various state of the art methods of obtaining a relativelyaccurate map of the seafloor have been proposed and include: (I)multibeam sonar with transducers attached to hull of surface vessel; (2)sidelooking sonar with transducers attached to towed platform flying ata short distance above bottom; and (3) photogrammetric camera or sterocamera-pair attached to a low-flying manned submersible. Relativelysmall bottom areas of about 200 feet by 200 feet are mostcost'effectively mapped by the towed-fish sidesonar method. A majorproblem is the measurement of true xyz coordinates of bottom pointsrelative to a bottom benchmark.

At least two techniques are presently being used to measure xyzcoordinates of seafloor points from sidescan sonar records. One methoduses a pair of stacked transducers operated simultaneously and connectedto the receiving electronics so that the difference in the signals fromthe two transducers is recorded in time. This constitutes aninterferometer system. The other technique uses two separate side-sonarrecords obtained with a single transducer; the records are generated byscanning the bottom area twice from two different off-bottom heights.Both techniques require considerableron-shore processing of data toconstruct a contour map of the scanned seafloor area.

A major difficulty in measuring xyz coordinates of bottom points, usinga towed side-scan system, is determiningthe position of the towed systemrelative to a bottom benchmark. This is done by using a longbaselinetransponder network, which consists of three or more bottom units, eachrequiring a self-contained power supply and complex electroniccircuitry. Baseline engths are measured by a surveying-in operationwhich requires about a day of data taking and, if oneboard computerfacilities are lacking, another day to process the data.

State-of-the-art methods of implanting objects on the seafloor atprecisely determined points also utilize transponder system, either thelong baseline or short baseline types. In either type of transpondersystem, one or more bottom units are required, again each unit having aself-contained power supply and complex electronics. Echo rangingmethods of positioning objects on the ocean floor uses a single 360scanning transducer which does not measure true elevation and lateraldistance of position.

SUMMARY OF THE INVENTION Briefly, the present invention includes a pairof pulsed sonar transducers which are mounted one above the other on astationary mounting frame positioned on the ocean floor. The pair oftransducers are rotated as a single unit but are spaced sufficiently toproduce a three dimensional figure when viewed stereoptically.

Because this invention has the scanning transducers mounted rigidlytogether on the same support structure, the problems associated withseparately towed sonar transducers will be eliminated. Techniques ofobtaining radar stereo imagery utilize a pair of radargrams obtainedfrom a single radar transducer flown along separated flight paths, theseflight paths are parallel and are oriented relative to each other so asto maximize the stereo effect. If the target area is scanned fromopposite side, maximum parallax is obtained; however, the left eye maynot recognize a scanned object as the same one seen with the right eyebecause of distortion. Such distortion is found in radar imagery as wellas sonar imagery because radar also measures distance in the same waythat sonar measures distance. Hence, in radar stereo it has been foundthat same side scanning produces better stereo imagery than oppositeside scanning. The invention described in this disclosure has theadvantage of same side scanning because the transducers are mounted oneabove the other on a vertical axis.

Therfore, an object of this invention is to provide a device adapted togenerate a pair of sonar records in one operation, which records can beused to produce a three-dimensional picture.

Another object of the invention is to provide a device capable ofmeasuring true xyz coordinates of natural ocean bottom features.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings. whereinBRIEF DESCRIPTION OF THE DRAWING FIG. 1 is schematic view of the presentstereo sonar system during a scanning operation.

FIG. 2 is a view of the transducers and illustrates the beam geometryfor near-field operation.

FIG. 3 is a schematic view of sonar imagery of the cylindrical object.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawingswherein like numbers refer to like parts it is seen that the inventionis a stereo sonar system 10 including a pair of fan-beam pulsed sonartransducers 12 and 14 mounted one above the other in the same axis onelement 44 which is part of a conventional stationary bottom supportframe 16 best shown in FIG. 1. The latter possessing adjustable feet 40in order to level the system 10 in the event of uneven aqueous terrain.The pair of transducers 12 and 14 is conventionally rotated as a singleunit by motor 42 actuated via a signal sent from the surface ship. Thevertical separation of the transducers 12 and 14 is large enough toproduce a pair of sonar records which will blend together in astereo-viewing system to present a three dimensional picture of acircular area of the ocean floor.

The entire system 10 is lowered to the ocean floor from a surface shipthrough a series of cables notably 26 and 28 attached to supportstructure 16 at 30 and 32 respectively. The cables are maintained inparallel position by separator bar 34 above which the cables are joinedby ring 36 to the surface ship cable 38.

As an example of a specific operational scheme. sonar parameters arechosen so that the scanned circular area is within the near field ofeach transducer, so

that the beam shape is a wedge having constant width equal to thetransducer length, as shown in FIG. 2'. For a scanned area of 100 footradius, a suitable transducer length, L is 1.5 ft. (FIG. 1). If thesound frequency is 300 kHz, a representative value for high-resolutionside-looking sonar systems, then the range, r,,, of the near field isaround l35 feet, as computed from the relation:

r L /lt where A is wavelength, l.67 ft. With the upper transducer about33 feet above bottom, the maximum echo range for a 100 feet radium areais under 135 feet.

The elevation beam angle of each transducer is around 60 and thedepression angle about 8, hence a 360 scan with the rotating transducerpair 12 and 14 will cover most of the circular area centered at the axisof the system. A circle of radius about 6 feet will be excluded. Becausethe transducer pair 12 and 14 is mounted on a rigid, stationarystructure 16, the stereo effect will be attained even though the360-scanning operation takes a relatively long time, as compared to thetime it takes to produce a pair of stereo photographs.

The area to be mapped or to contain emplaced objects, is scanned asector at a time; the angular width of a sector for the near-field caseis roughly equal to the ratio of transducer length to radius of area.The maximum round-trip travel time for a single pulse is around 42milliseconds for a 100 ft radius circle and transducer height of 33feet; and the angular width of a sector insonified by a single pulse isapproximately 1.1 degrees. Hence, assuming 360 sectors will be isonifiedin one 360 scan, we see that a single scan will take at least 15seconds. For coverage of a large circular area, far-field operationwould be included and a different set of sonar parameters chosen.

Because sonar detects range, instead of angle (as is done by aphotographic system), the sonar image of an object scanned by a fan beamwill not have the appearance of the image obtained photographically.This situation is illustrated in FIG. 3 which shows the sonar image 18of a cylinder. The image 18 is a kind ofinsideout cylinder, resultingfrom the fact that contour 20 (FIG. 3) is nearer to the transducer thancontour 22 (FIG. 3); this situation causes the bottom of the cylinder toappear between contours 20 and 24. The threedimensional picture,obtained from blending the imagery from each sonar transducer in astereo viewer, will therefore, not be a conventional three-dimensionalrepresentation of the bottom area or a set of objects within this area.The important fact is, however, that the picture will bethree-dimensional and will make possible the construction ofa map withpoints identified by true xyz coordinates.

Another use of the invention is the positioning of objects relative toan array of seafloor benchmarks. If natural seafloor features arepresent, such as outcroppings or faults, they can also be used asbenchmarks in the positioning operation. Before the objects areemployed, a sonar stereo pair of records are obtained of the area whereemplacements will be made.

One of the principle features of this invention is the fact that a pairof sonar records is generated in one operation, and these records can beused to produce a three-dimensional picture from which topography can beimmediately recognized. In existing methods of mapping by side-lookingsonar, a considerable amount of on-shore labor is required to match thetwo records obtained in separate scanning operations. The matching ofcorresponding points on two records is absolutely essential in order tocompute true xyz coordinates; and using the stereo scanning system willresult in the identification of bottom features which would beoverlooked in the older method of mapping. With a suitably designedstereo viewing system, the entire measurement process can be conductedaboard ship in a relatively short time.

Another feature of this invention is in its use as a positioning system.Existing systems use transponders or a single 360 echo-ranging sonardevice. Such a device cannot measure true xyz coordinates ofnatural'bottom" features or previously emplaced emplaced sonarbenchmarks. The rotating stereo sonar system has the advantage ofyielding true coordinates and, hence, more precise positioning. Also,the stereo system will permit easier recognition of natural bottomfeatures, than existing echo-ranging techniques, and hence will reducethe time required to relocate the seafloor site where the positioningoperation is to take place.

Within the present concept for producing a pair of stereo sonar recordswould be one in which a single fan-beam transducer is operated at twodifferent elevations on the same vertical axis. The two sonar imageswould produce a three-dimensional picture having a quality equal to thatof the simultaneously operated transducer pair; however, because two 360scan would be required, the time of observation would be approximatelydoubled. The stereo blending of the records would still be possible byusing conventional video-tape methods.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A stereo sonar system for both accurate mapping of an aqueous floorand positioning of objects on said floor comprising:

a supporting framework;

a pair of fan-beam pulsed sonar transducers mounted one above the otherin spaced relationship on said framework and;

means for rotating said transducer as a unit;

said transducers adapted to be rotated together but being sufficientlyspaced to produce a pair of sonar records in one operation which areused to produce a three-dimensional figure when stereo viewed.

2. The stereo sonar system of claim 1 wherein:

the transducers are mounted in spaced vertical relationship on the sameaxis.

3. The stereo sonar system of claim 2 wherein:

said supporting framework is adjustable to provide leveling action inthe event the aqueous floor is uneven.

4. A stereo sonar system for both accurate mapping of an underwaterfloor and positioning of objects on said floor comprising:

a supporting framework;

a single fan-beam pulsed sonar transducer mounted on said framework;

means for rotating said transducer;

said single transducer being operated at two different elevations on thesame vertical axis while being rotated to obtain records of twodifferent sonar images which are used to produce a threedimensionsalpicture of a circular area of said floor when both images are stereoviewed.

1. A stereo sonar system for both accurate mapping of an aqueous floorand positioning of objects on said floor comprising: a supportingframework; a pair of fan-beam pulsed sonar transducers mounted one abovethe other in spaced relationship on said framework and; means forrotating said transducer as a unit; said transducers adapted to berotated together but being sufficiently spaced to produce a pair ofsonar records in one operation which are used to produce athree-dimensional figure when stereo viewed.
 2. The stereo sonar systemof claim 1 wherein: the transducers are mounted in spaced verticalrelationship on the same axis.
 3. The stereo sonar system of claim 2wherein: said supporting framework is adjustable to provide levelingaction in the event the aqueous floor is uneven.
 4. A stereo sonarsystem for both accurate mapping of an underwater floor and positioningof objects on said floor comprising: a supporting framework; a singlefan-beam pulsed sonar transducer mounted on said framework; means forrotating said transducer; said single transducer being operated at twodifferent elevations on the same vertical axis while being rotated toobtain records of two different sonar images which are used to produce athree-dimensionsal picture of a circular area of said floor when bothimages are stereo viewed.